The present invention relates to systems for electronically alerting people using portable devices and transmitting short messages to those devices, for example a telephone number that should be called. This field of one-way radio communications of short messages is known as paging.
Conventional paging systems are one-way communications systems in which the portable unit consists of only a receiver. Paging services are, however, facing increasing competition from two-way cellular phone systems, as the latest cellular phones are small, of low cost, and offer long battery life and voice communications. The paging services market is therefore responding to competition by expressing an interest in moving towards two-way communications services, such as short message services or voice mailboxes in the network, which services have to be triggered to replay the centrally stored message by the paging unit, thereby necessitating communication in the reverse direction, i.e., from the paging unit to the network.
However, there are many technical problems in providing reverse communication from a paging unit to a base station. Paging broadcast transmitters are typically of high power, for example 100 to 250 watts, in order to compensate for the low efficiency of the paging receiver antenna which is usually worn close to the user's body. The provision of capability for pagers to communicate in the reverse direction is thus seriously impeded by the need for a similarly high transmitter power to close the link in the reverse direction.
Conventional systems do exist which provide two-way radio communication from a portable unit back to a base unit with much lower portable transmitter power than used in the forward, i.e. base-to-portable, direction. A landmobile radio system such as the EDACS system manufactured by Ericsson Inc. in Lynchburg, Va. is an example of such a system. In these conventional systems, the reverse link is closed by providing multiple, distributed, base station receiving sites so that the range from a portable unit to the nearest receiving site is much less than the range from the base transmitter to the portable, thus allowing the portable transmitter power to be much lower than the base transmitter power. However, the provision of multiple receiver sites can be costly.
Conventional systems also contain examples of two-way communications from a portable unit to a base unit using lower power than that used in the forward direction, and without using a much greater number of fixed receiver sites than fixed transmitter sites. A cellular phone system is an example of such a system. Each site, known as a cell-site, comprises at least one transmitter and at least one receiver. All sites are thus both transmitter sites and receiver sites. To allow the portable unit to employ lower power than the base unit while communicating over the same distance, the fixed receiving system often provides spatial-diversity reception by using two, spaced-apart receiving antennas. This provides a gain of 7 dB when signals are subject to Rayleigh fading, allowing the portable unit power to be, in principle, five times lower than the base power.
Conventional cellular systems also include the use of directional antennas at the base station. Typically, the base station antenna includes three directional antennas placed around a mast at 120 degree intervals and each covering three 120 degree sectors or cells. A sectorized cellular system can be regarded as having collected together the base sites of three adjoining cells, the cells then being illuminated from their common edge instead of their separate centers, thus reducing the number of sites by three to save real estate costs. The extra communications distance that the base must serve, i.e. from cell edge to cell edge instead of center-to-edge, is accommodated by the extra directive gain of the 120-degree sector antenna as compared to the omni-directional antenna that would be used in the case of central illumination. Sectorization in cellular systems is therefore a way of providing the same performance with reduced real estate costs.
The antenna direction or sector to be used for serving a particular mobile phone is determined at call set-up time and adequate time is available to establish the sector to be used, due to the relatively long duration of a telephone call. The same antenna direction is used for transmission as well as for reception in cellular systems. This solution does not, however, function for a paging system which broadcasts a paging message over the full 360 degree azimuth in which a portable unit may be located and which system does not know in advance what antenna direction to use either for transmission or reception. In a paging system, messages are typically too short to execute a call set-up procedure similar to that employed in making a cellular telephone call, and which enable antenna directivity to be properly selected to serve the call.
The above-incorporated by reference parent patent applications disclose ways and systems to enhance reception antenna directivity by the use of antenna arrays. The parent applications also disclose employing known symbol patterns transmitted by mobile transmitters at the base receiver site in order to determine the optimum coefficients with which signals from the antenna elements can be combined to enhance reception. Moreover, these applications also describe using signals received to determine phase and amplitude errors between array elements and to correct same on a long-term basis. These techniques are further developed below to overcome the deficiencies of conventional paging systems that hinder implementation of a two-way paging system.